And again I ask you. Is it possible? Is it possible? Your only answer is Yes, under the normal rules.
And to avoid these fate, you have to use time travel, wishes and many shenanigans.
You assume that characters win every fight and never flee. Nothing can be further from the truth.
And you used Time Travel as an excuse for why losing a Kingdom is nothing more than a mild inconvenience. So, if I must accept that a kingdom destroyed by the forces of evil is nothing more than a mild inconvience due to time travel being possible, why is death suddenly so permanent and forever if I can use "many shenangins" including basic spells everyone is aware of and will prepare to combat death?
You present a double standard, then act like being called on it is some grave sin.
Character will flee and regroup. Sometimes, it means that the dead character is left behind. Leaving time to either being raised into an undead or simply rise as one. You can be swallowed, be still in the area of a fireball cast by friendly or unfriendly fire as you died. How many HP does a corpse have. A medium barrel has about 18 hp. A fireball will destroy it as it will destroy the corpse of your friend. That is unless you rule that dead character are totally immune to all damage. And that is assuming simple normal play. Once again, not every fight can be won. The dragon will not stop to use its breath because the corpse of a character happens to be there. Heck, that fire can melt armor! Imagine a single corpse. Or being swallowed, the purple worm isn't the only creature to swallow hole. There are some frogs that can do that too.
Faced with overwhelming odds, characters will flee, leaving their dead friend there. Unless that in your games, characters are so powerful that they never have to flee... It would explain a lot of things and positions you have.
PCs rarely flee and leave the dead corpse of their friends behind... Because they want to raise their dead friend and prevent them from staying dead. They have gone to incredible lengths to grab that body or at least a piece of it,
Very few enemies swallow dead corpses.
Also, as for how much health a corpse has... this is something that is never covered in the game. There are zero rules for damaging corpses. It is equally RAW that since you can't die due to death from massive damage until you have taken your negative hp in damage then the corpse can't be destroyed until it takes the negative hp as damage. Which is far more than 18.
And, yea, I generally don't bother tracking damage done to corpses if they were caught in an AOE. I also don't rule that fireball destroys stone floors (10x10 object, resilient, 27 hp) if they roll 30 damage. It is questionable if fire damage could even prevent revify from working, because they aren't missing any limbs and they didn't die of old age, which are the only restrictions per RAW.
Also
Once undead, your creature type is now undead. Be you a corpse, you will be revivified as what you were previously, an undead. Why do you think that such spells as raise dead specify it can not target undead type? Heck, to remove that fate, you need True Resurection. The only spell that specifies it!
Um... because you can't use Raise Dead to bring back an actively undead zombie, or a Revenant, or a Death Knight, or reverse Lichdom. There is literally nothing that says the corpse of a zombie is still a zombie and not just a corpse. Again, you seem very intent on these houserules for someone who is so adamant that you are the only one following RAW.
In fact, there is a line in Curse of Strahd about killing a Vampire in the church and raising them back as a human. No True Resurrection required
Also, you said: You've also accused me of picking on your "weak" examples, when... I've addressed all your examples? Seems like a rather strange claim that I only picked on the ones easy to disprove, which was all the ones you pointed out."
Of course I will admit that my examples were weak. Not doing it would be disingenuous and flattly be bad faith. And I told you why: "I was not near my books". But the spirit of these examples are still relevant. Keep in mind that I do not fear being proven wrong when I am wrong. But this time, I am not. Death is truly the only thing that can be permanent. All the rest are just setbacks that will spring more adventures. These setbacks might be hard to swallow and accept. But they can be overcome. What does not kill you, makes you stronger has never been truer than in fantasy.
And as I've now shown repeatedly, Death too is merely a setback and not permanent. Since your definition of "permanent" involves "can't be undone via time travel". So, nothing in DnD is permanent.